1. Field of the Invention
This invention relates to an injection-molded product which is greatly improved in terms of its appearance (or clear appearance of a coating applied on it) and its low linear expandability, and possesses excellent low-temperature impact resistance and flexibility.
2. Background Art
Automobile parts have now been much more increasingly made of plastics with a view to cutting down fuel costs by weight saving, and so various plastic materials have been used not only for interior parts such as instrumental panels, console boxes, glove compartments, steering wheels and trims and exterior parts such as laces, lamp housings, front grills, mud guards and side bumpers, but also for bumpers, facias and (parts of) bodies heretofore made of metals.
Plastics used for such automobile parts include, for instance, RIM/urethane, composite polypropylene and polymer alloys such as polycarbonate & ABS. In particular, materials used for bumpers and facias are currently broken down into two types, i.e., materials of low rigidity having the property of absorbing impact energy primarily by their deformation and, then, recovering to their original form and the other type of materials of high rigidity which are inferior in deformation recovery properties but are lighter in weight and less expensive.
Of these materials, the former materials are represented by RIM/urethane and polypropylene blended with partially crosslinked ethylene/propylene copolymer rubber (EPR), and generally have a modulus in flexure of about 2,000 to 5,000 kg/cm.sup.2. The latter materials, represented by polypropylene blended with EPR & inorganic filler, generally have a modulus-in-flexure of about 8,000 to 12,000 kg/cm.sup.2. These materials are now widely used because of low cost and high productivity upon processing into articles by injection molding.
Specific examples of the low-rigidity type materials are polypropylene compositions comprising partially crosslinked ethylene/propylene copolymer rubber, oil and so on which are prepared by methods as disclosed in Japanese Patent Kokai Publication Nos. 53(1978)-145857, 54(1979)-16554 and 57(1982)-135847, and polypropylene compositions comprising non-crosslinked ethylene/propylene copolymer rubber and high-density polyethylene which are prepared by a method as described in Japanese Patent Kokai Publication No. 56(1981)-106948.
Such low-rigidity type materials are also exemplified by, for instance, polypropylene compositions comprising non-crosslinked ethylene/propylene copolymer rubber and oil, which are prepared by methods as disclosed in Japanese Patent Kokai Publication Nos. 52(1977)-23148, 52(1977)-12650 and 63(1988)-146951.
However, it has been found that the low-rigidity type materials as prepared by the methods disclosed in the above Kokai publications are often lacking in well-balanced quality in view of low-temperature impact resistance and appearance, when injection-molded, and are less than satisfactory when recent car design, esp., the generally uniform flatness of bodies and a coating finish close to sheet metal working are taken in account, because their coefficient of linear expansion is as high as about 15.times.10.sup.-5 /.degree.C. and the clearness of coatings applied on them becomes unsatisfactory. Thus, such low-rigidity materials are prepared by complicated steps including, esp., a crosslinking step so that they are likely to have an ununiform flowability. In consequence, they tend to become unsatisfactory in view of appearance and low-temperature impact resistance, when injection-molded.
The high-rigidity type materials prepared by the above Kokai publications are well-balanced in terms of such properties as low-temperature impact resistance, heat resistance and moldability, but are not satisfactory if the generally uniform flatness of bodies and a coating finish close to sheet metal working are in mind, because like the low-rigidity type materials, they are unsatisfactory in terms of the clearness of coatings applied on them and their coefficient of linear expansion.
In order to prepare materials of low linear expandability, on the other hand, it is a general technique to fill resins with sheet-like or fibrous fillers. As set forth in, for instance, Japanese Patent Kokai Publication Nos. 61(1986)-21145 and 63(1988)-57653, it is known to use glass fibers, talc, whiskers or the like as fillers. In order to obtain materials of sufficiently low linear expandability, however, it is required to add large amounts of fillers. This results in deterioration in flowability and, hence, the appearance (including the clearness of coatings applied on them) and low-temperature impact resistance of molded products.